Fiber optical cables are widely known in the art as cables containing at least one optical fiber, but generally containing a plurality of optical fibers. Typical construction of optical fiber cables includes an external polymer jacket that is extruded over the fibers for protection. Within this extruded outer jacket, one or more tubes are contained therein, each of which having one or more optical fibers inside. Fiber optic cables with more than one tube are referred to as multi-tube optical fiber cables.
In addition to the optical fiber containing tubes, the cables also typically contain strength and protective elements in proximity to the tubes. These strength and protective elements may include glass or fiber reinforced plastic rods or other strength fibers such as flame resistant aramid fibers. Also, water swellable tapes or filaments can be added to prevent water ingress, avoiding water exposure to cable elements and downstream equipment.
Of the types of optical fiber cables available, single tube fiber optic cables are typically constructed with a single tube containing one or more fibers, where the tube is located in the center axis of the cable. However, this construction is limited in capacity, as it only has a single tube.
Multi-tube fiber optic cables increase the capacity by adding more tubes, however, there are a number of mechanical drawbacks that occur during both production of and installation of multi-tube cables.
When producing multi-tube fiber optic cables, it is necessary to stabilize the geometry of the tubes within the cable jacket so that the position of the tubes maintains a regular geometry along the length cable. This is necessary because if the tubes were simply allowed to freely float within the jacket it is possible that the length of one tube with respect to another tube or with respect to the jacket could begin to deviate, causing the fiber containing tubes to have a significantly different lengths than one another or the cable itself.
Also, if the tubes do not exhibit a regular geometry within the jacket, there can be problems if the cable experiences mechanical stresses from bending or temperature shrinkage. In severe cases, a reduction in the efficiency of the fibers in the tubes may result as the fibers may undergo widely varying physical stresses, particularly if one of the tubes is entirely located against the outer jacket. Furthermore, such irregular location of the tubes and supporting strength elements within the jacket may simply cause physical difficulties or deformities in the optical fibers leading to micro or macro bending of a individual optical fiber's light reflective channel (core) resulting in light (power) loss or attenuation.
To address this problem, prior art solutions in multi-tube fiber optic cables add an additional stranding step where tubes and strength members are stranded or cross bound with binding yarns or tapes so as to hold them in a particular regular geometry such as helical or oscillated arrangement. The regular oscillated or helical geometry of the tubes and strength elements improves the mechanical characteristics by ensuring equal and excess lengths of tubes relative to the cable jacket, allowing for better handling of mechanical stresses during spooling, uncoiling and installation.
However, this additional step is not only costly and time consuming but it also adds significant weight to the final cable. Stranding typically requires a geometrically correct grouping of elements, 4 or greater in number, to be stranded about a central element where less elements may be preferred.
Another method available to stabilize the location of the tubes within the cable jacket is to use pressure extrusion, where the outer jacket diameter is the same, but the inner side of the jacket is pressurized against the internal tubes and strength elements, thus filling in empty spaces and holding them in place. However, drawing tubes in a particular geometry while simultaneously pressure extruding the jacket is extremely difficult from a mechanical perspective because the pressure of the plastic from the extrusion head simply pushes the lighter tubes and strength elements out of any desired arrangement.